1. Field of the Invention
The present invention relates to a balun (balanced-to-unbalanced transformer) that connects a balanced circuit and an unbalanced circuit together, for example, for transforming the impedance of transmission lines in a high frequency of UHF band or higher.
2. Description of the Related Art
As a conventional balun, a technique disclosed in JP-A-2002-43882 is known. In the balun disclosed in this document, a band-pass filter is connected to the terminal of an unbalanced line of the balun. As an alternative example, a technique disclosed in JP-A-2002-271036 is known. The circuit of this technique is provided, as shown in FIG. 22, with a balun 20 which is composed of an unbalanced transmission line 25 and a pair of balanced transmission lines 26, a second-order low-pass filter 27 which is composed of an L and a C connected to the unbalanced transmission line, and a matching circuit 22 of diplexer which is composed of a third-order high-pass filter 28 that is composed of an L and three Cs.
In the conventional balun disclosed in JP-A-2002-43882, since a band-pass filter is connected, it is difficult to make the band broader. Also, in the matching circuit 22 of diplexer disclosed in JP-A-2002-217036, since passing bands of the low-pass filter 27 and the high-pass filter 28 are generally designed not to overlap each other, the lower band side of the balun 20 becomes a stopping band to the high-pass filter 28 and the higher band side of the balun becomes a stopping band to the low-pass filter 27, therefore, it is unsuitable to make the band broader, similarly to JP-A-2002-43882 (See FIG. 23).
Furthermore, in recent years, ultra wide band communication equipment (e.g. mobile phone and wireless LAN) is getting attention, and its standard desires that such equipment transmits an ultra wide band of about 3 GHz to 10 GHz with a flat characteristic. Accordingly, to realize an ultra wide band filter or balun, the band must be made broader than that used before. It causes another problem, however, that such a filter or balun tends to become larger in size if the ultra wide band is designed by a commonly known method. Therefore, it is unsuitable as a device expected to be built in equipment, such as mobile phones, that are required to be microminiaturized.